Permeable membrane systems are widely recognized as a convenient and highly advantageous means for effecting gas separation on a relatively large scale. Such systems typically incorporate one or more permeable membranes having suitable permeability and selectivity characteristics for separating one or more component gases from a compressor-driven gas mixture feed. Such systems are used, e.g., in the separation of nitrogen, or oxygen (or both) from air. The retentate component (i.e. the gas which does not permeate the membrane) is typically the product, although it is possible to obtain (instead or in addition) an enriched or purified permeate product by providing one or more membrane separation stages. Membranes useful for this purpose include, without limitation, membranes permeable to oxygen, carbon dioxide, moisture, hydrogen, helium and the like.
Membrane separation performance is typically sensitive to both temperature and pressure. As operating temperature increases, membrane permeability increases as well, permitting more product to pass through the membrane. However, while membrane permeability increases with temperature, its selectivity decreases, thus requiring additional feed gas flow to maintain product (retentate) purity. As operating pressure increases, product purity rises due to an increased driving force pushing more gas through the membrane.
Striving to produce a relatively pure product at a specific flowrate within an expected range of ambient environmental conditions (temperature, pressure, humidity and air quality), conventional membrane system designs typically match compressor capacity with a required amount of membrane area at specific process conditions. Proper matching of the compressor to the membrane area is important in achieving cost-effective operation of the system. Also important is the balancing between capital investment (compressor size and membrane area) and power consumption costs.
In an effort to match compressors to membranes, those skilled in the art have realized that environmental parameters such as ambient temperature, relative humidity and barometric pressure all have varying effects on compressor capacity. Thus, compressors selected as feed gas generators are typically configured to operate at maximum volumetric capacity under worst case conditions. With these criteria in mind, the compressor's capacity is usually fully utilized at a design ambient temperature "T.sub.a ", typically the maximum ambient temperature within the expected range.
Conventional membrane systems operate under conditions that avoid the presence of any condensing liquids. Liquids, especially hydrocarbons, degrade membrane performance. For this reason, a predetermined minimum operating temperature (T.sub.O) is typically selected to ensure superheating of all condensable feed components (humidity and pollutants) at low ambient temperatures. However, when the ambient temperature rises above T.sub.a, the system operating temperature must increase above T.sub.O to maintain all components of the fluid mixture in a superheated condition and to avoid performance degradation of the membrane. In such a situation, the compressor capacity is fully utilized although the product supply rate falls.
In situations where the ambient temperature falls below T.sub.a, the operating temperature of a conventional membrane system is typically maintained at the minimum threshold level T.sub.O. This correspondingly affects the permeability of the membrane to a feed supply rate associated with that temperature. Because the density of the feed gas is higher at lower ambient environmental temperatures, the feed compressor has additional unused capacity. However, this unused capacity has not been exploited in the prior art. Instead of tapping into the increased capacity, conventional membrane systems often employ a "turn down" or bypass mode wherein the compressor output is controlled (decreased) to match the decrease in membrane permeability, which is also associated with the lower temperature. The prior art has failed to realize that "turndown" under these conditions actually results in higher production cost over a period of time.
Conventional gas separation membrane system designs that "turndown" the feed gas compressor during low demand periods often employ a controller responsive to certain demand parameters to increase or decrease feed gas flow from the compressor. Illustrative of such designs is U.S. Pat. No. 5,281,253, assigned to the assignee of the present invention. The controller typically includes means for monitoring and sensing at least one, and preferably all of the pressure, flowrate and product purity at the outlet line of the membrane system. Also included with the controller is a capacity control device to vary the compressor output. When changes in the monitored parameters occur, indicating a reduction in product demand, the compressor output is decreased to reduce power consumption. While this design is beneficial for its intended purpose, it fails to act upon process parameter deviations resulting from changes in process operating temperature and thus fails to take advantage of the virtually cost-free excess capacity in compressor feed gas flow associated with a decrease in ambient temperature.
Therefore, those skilled in the art have failed to recognize the need to provide a membrane system controller and control method for controlling a permeable membrane system to utilize the additional compressor capacity available during advantageous changes in ambient temperature (i.e. during cold winter months).